[Misc] Add SPDX-FileCopyrightText (#19100)

Signed-off-by: simon-mo <simon.mo@hey.com>

tests/async_engine/api_server_async_engine.py

@@ -1,4 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 """vllm.entrypoints.api_server with some extra logging for testing."""
 from collections.abc import Iterable
 from typing import Any